1. Field of the Invention
The invention relates primarily to bicycles, but also to any other human powered vehicles, watercraft, or exercise devices which utilize a foot pedal or handgrip for the operator to convert rotational motion and/or linear motion of his feet or hands, into work, in order to activate the device.
2. Description of Prior Art
Bicycle crank arms in general, provide a means for physically connecting the foot pedals of the bicycle to the crankshaft of the bicycle. In some cases, one of the crank arms is also attached to a chain sprocket, or a set of axially concentric sprockets, which drives a chain for the purpose of transmitting power to the drive wheel of the bicycle. Whether the sprocket is attached directly to the crankshaft or spindle, or indirectly to the spindle through one of the crank arm assemblies, the crank arms enable force exerted on the foot pedals to be transferred into power to propel the bicycle as the foot pedals sweep through each stroke.
Bicycles have evolved from their earliest designs with a pair of bilaterally symmetric and inversely synchronized foot pedals, each connected to the crankshaft or spindle of the vehicle by a generally straight crank arm. As bicycle consumers have come to put more emphasis on light weight and performance, bicycle manufacturers have endeavored to manufacture lighter and better performing bicycles. This quest has led many manufacturers to utilize computer aided design techniques and exotic materials in the creation of their products. Today, the weights of many components on bicycles are usually communicated in gram units, because the emphasis on weight reduction is so great that the units of pounds and ounces are insufficiently explicit for many consumers.
One of the largest concentrations of structural material in a typical bicycle is in the crank arm. That often corresponds to one of the largest concentrations of weight on a bicycle, despite some manufacturers"" use of exotic lightweight materials at this location. Reducing weight by using such materials usually leads to a significant cost penalties or other tradeoffs.
This invention replaces the double ended crank arm design currently used on bicycles, with a triangulated crank structure. Triangulation is accomplished by replacing a straight bar type structure that connects the spindle to the pedal shaft end of a crank arm, with a split structure that has two separate tube segments, spaced away from a line between the spindle end and the corresponding pedal shaft attachment location, that line being the neutral axis of the structure. During the rider""s power stroke, one such tube segment would be mostly under tension while the other would be mostly under compression. This largely eliminates high bending stresses associated with the straight crank design. It does so by moving structural material much further away from the neutral axis of the crank than is possible with a straight crank design.
Mechanical triangulation when applied to bicycle cranks permits a maximum structural efficiency defined as the minimum amount of structural material possible to support a given load. The inherent structural efficiency problem in a triangulated design is the requirement for multiple structural segments. Furthermore, the shortest distance between the spindle end and the pedal shaft attachment means, and therefore the shortest total length of structural material required to connect them, is the straight segment of the prior art.
This invention preferably uses hollow tubes to save weight in the multiple segments. The hollow tubes are structurally optimized to resist both torsion loads due to the offset pedal, and the lateral bending loads perpendicular to the applied force that the tubes experience during cyclical power transmission peaks. The hollow tubes also approach an ideal design for resisting crank deflection when the rider""s weight is pushing on the pedal while at or near the top or bottom of the stroke. Also, this invention preferably uses only two structural segments or struts to connect the hollow tubes to the spindle and/or sprocket assembly. Lastly, the crank structure on the same side of the vehicle as the drive sprocket uses two structural segments or struts to connect the structure efficiently to four sprocket bolts on a five bolt sprocket, or to two sprocket bolts on a four bolt sprocket.
The primary object of the invention is to improve structural efficiency of bicycle cranks, thereby decreasing weight without any reduction in strength or stiffness. The invention can provide bicycle cranks with both strength enhancement and weight reduction benefits.